1. Field of the Invention
The present invention relates to an imaging device capable of imaging objects in a maximum of the omniazimuthal angle of 360 degrees and used in the field of visual systems such as, for example, surveillance cameras.
2. Description of the Related Art
Recently in the field of visual systems such as, for example, surveillance cameras, various attempts have been made to allow a camera to perform monitoring operations conventionally conducted by the human eye, by combining the camera with a computer.
A generally used camera, which has a limited viewing angle, is not suitable for such applications. Therefore, cameras using fish-eye lenses or other wide-angle lenses have been developed. For example, in the field of movable robots, the use of convex mirrors having a shape of solid of revolution (such as conical mirrors), spherical mirrors or the like, have been actively studied. (Hereinafter, such convex mirrors will be referred to as “convex rotatable mirrors”.) According to systems studied in this field, an optical image of a viewing angle of 360 degrees is taken by a convex rotatable mirror, then the optical image is converted into a video image, and the video image is further converted into a desired image by a computer.
FIG. 9 is a projection figure of an imaging device 90 using a conventional convex rotational mirror. The imaging device 90 includes a convex rotational mirror unit 91. The convex rotational mirror unit 91 includes a generally disc-shaped base 92 and a convex rotational mirror 93 provided on a surface of the base 92. The imaging device 90 further includes a generally cylindrical optical member 94. The optical member 94 is open toward the convex rotational mirror unit 91 and covers the surface of the base 92 and the convex rotational mirror 93. The optical member 94 holds the convex rotational mirror unit 91 and is formed of a light-transmissive material. An inner circumferential surface of the optical member 94 and the convex rotational mirror 93 interpose a hollow space therebetween. The optical member 94 has a thickness which is sufficiently thin to allow light which is incident on an outer circumferential surface of the optical member 94 to be transmitted through the optical member 94, so that it is proximately parallel to light which is directed toward the convex rotational mirror 93 from the inner circumferential surface of the optical member 94.
A generally cylindrical imaging mechanism 98 is attached at the opposite side to the convex rotational mirror unit 91, with the optical member 94 interposed therebetween. The imaging mechanism 98 includes a lens 99 facing an opening of the optical member 94, which is formed on the opposite side to the convex rotational mirror unit 91, and an imaging section 90 provided on the opposite side to the optical member 94, with the lens 99 interposed therebetween. The imaging section 90 is connected to a signal processing section 88 provided for adjusting distortion of an image taken by the imaging mechanism 98.
As described above, the light-transmissive optical member 94 is used for holding the convex rotational mirror unit 91, and thus a separate holding member is not provided. The reason is that if a separate holding member is provided for holding the convex rotational mirror unit 91, an image of the holding member itself would be taken and so would be a part of an image taken by the imaging mechanism 98.
The imaging device 90 having the above-described structure operates as follows.
Light 71 is incident on the outer circumferential surface of the light-transmissive optical member 94 and is transmitted through the optical member 94. While being transmitted through the optical member 94, the incident light 71 is refracted twice (not shown) so as to become light 72. The light 72 is directed from the inner circumferential surface of the optical member 94 toward the convex rotational mirror 93 through the hollow space between the optical member 94 and the convex rotational mirror 93. Then, the light 72 is reflected by the convex rotational mirror 93 and is directed toward the imaging mechanism 98 as reflected light 81. The reflected light 81 is transmitted through the lens 99 of the imaging mechanism 98 and is incident on the imaging section 90. The imaging section 90 converts the reflected light 81 into an image signal representing an image and outputs the image signal to the signal processing section 88. The signal processing section 88 processes the received image signal so as to adjust the distortion of the image.
Light 73, which is incident on the outer circumferential surface of the light-transmissive optical member 94 from the opposite direction to the light 71, is transmitted through the optical member 94. While being transmitted through the optical member 94, the incident light 73 is refracted twice (not shown) so as to become light 74. The light 74 is directed from a portion of the inner circumferential surface of the optical member 94 toward another portion of the inner circumferential surface of the optical member 94 opposite the portion from which the light 74 is directed. The light 74 is reflected by the inner circumferential surface of the optical member 94 and is directed toward the convex rotational mirror 93 as reflected light 82. Thus, the reflected light 82 is superimposed on the light 72 directed toward the convex rotational mirror 93.
Normally, an image produced by the imaging device 90 representing the reflected light 81 reflected by the convex rotational mirror 93 is supposed to be based only on the incident light 71. When the reflected light 82 is superimposed on the light 72, however, an image produced by the imaging device 90 undesirably includes both an image based on the incident light 71 and another image based on the incident light 73 in a superimposed state.
Japanese Patent Publication (Kokai) No. heisei 11-174603 discloses an imaging device having a structure for solving this problem. In this publication, attention is paid to the fact that incident light reflected by an inner surface of an optical member and directed to a convex rotational mirror crosses the rotation axis of the convex rotational mirror. A rod-like member for shielding the incident light directed to the inner surface of the optical member is provided along the rotation axis of the convex rotational mirror. Thus, the incident light directed to the inner surface of the optical member is prevented from being reflected by the inner surface and thus prevented from being directed toward the convex rotational mirror.
However, the technology described in the above-mentioned publication has the following problems.
Since the rod-like member is required to be additionally provided, the structure of the imaging device is complicated and the number of production steps of the imaging device is increased.
Since the optical member is hollow, a structure is required to mechanically support the rod-like member provided along the rotation axis of the convex rotational mirror so that the rod-like member is not destroyed when the imaging device is actually used.